Design of FRPs in circular bridge column retrofits for ductility enhancement

Retrofit design of existing columns in buildings and bridge piers necessitates the accurate prediction of the deformation capacity of structures. In this study, an analytical model is proposed to estimate the ductility of potential plastic hinge regions of RC columns after a fiber reinforced polymer (FRP) retrofit. A simple bilinear stress–strain model that is capable of representing an FRP concrete response exhibiting softening at low confining pressures and hardening at higher confinement is proposed. This model is then employed in an approximate closed form sectional analysis of circular columns subjected to axial force and bending moment. Section ductilities that can be obtained as a result of FRP retrofit are expressed in terms of non-dimensional column parameters and confining pressure supplied by FRPs. The accuracy of the model to estimate the ductility of FRP retrofitted columns is verified by comparing model estimations with sectional analysis and test results. Subsequently, a simple non-iterative seismic retrofit design procedure using FRPs is established for circular bridge columns. Finally, parametric studies are conducted on a typical bridge column for different axial loads, reinforcement ratios and FRP amounts. Results are presented in the form of non-dimensional plots to aid engineers in the FRP retrofit design of columns for ductility enhancement.